PERFORMANCE EVALUATION OF HYBRID TECHNOLOGIES FOR TREATING TANNERY WASTEWATER

Abstract

The tanning industry is one of the important industries in India, which earns substantial foreign exchange through leather export. Leather production depends upon the various manufacturing processes that take place in the industry. The tanning industry releases its waste in the form of liquid, solid, and gas. The untreated release of tannery effluents containing high organic carbon, chemical oxygen demand (COD), biochemical oxygen demand (BOD) levels, trivalent chromium, sulphide, chlorides, calcium, magnesium, organics, and other toxic ingredients into the natural water bodies and open lands, affects the flora and fauna of the ecosystem. In recent years, due to stringent water quality regulations, extensive research is focused on upgrading water treatment technologies and developing processes that are economical and to deal with toxic and biologically refractory organic contaminants in wastewater. Research studies have suggested that the tannery wastewater effluent consists of complex compounds requiring a series of treatment processes to meet the desirable effluent discharge standards. Therefore, the research has been conducted on performance of hybrid technologies for the treatment of the tannery wastewater effluent to be able to reach to the desired effluent disposal standards up to the maximum extent. Objective 1 – Primary study was conducted mainly focussing on the removal of turbidity and total suspended solids from the tannery wastewater effluent using orange peel waste. However, the application of the orange peel waste was done using them as adsorbents. Orange peel waste was used in dehydrated and activated char form termed as orange peel powder and activated orange peel powder respectively. A comparative study was conducted on the degradation efficiencies obtained in terms of turbidity, total suspended solids (TSS) and chemical oxygen demand (COD) removal by using alum as a chemical coagulant, orange peel powder (OPP) and activated orange peel powder (AOPP) as naturally obtained adsorbents respectively. It was observed that the physico-chemical properties of the OPP and AOPP played an important role in the removal of the pollutants from tannery wastewater effluent. Scanning Electron Microscope (SEM) showed that the AOPP consists of a porous structure that leads to adsorption of pollutants over the surface. Brunauer–Emmett–Teller (BET) analysis resulted in surface area reduction of AOPP from 11.663m 2 /g to 1.289m 2 /g before and after adsorption correlating the experimental results of the study. Moreover, X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Total organic carbon (TOC) proved that the chemical properties also play an important role in degrading the pollutants present in TWE. The experimental analysis showed that AOPP as an adsorbent successfully removed turbidity, Total suspended solids, and COD by 98.43%, 97.65% and 49.85% respectively at optimized vi conditions of AOPP dosage – 2g/L, pH – 4, contact time – 30 min. Results proved that the application of AOPP as a pre-treatment of tannery wastewater effluent can be recommended for larger-scale implementation. Objective 2 – Tannery wastewater consists of several complex compounds that have poor biodegradability. The study further conducted experiments on the effluent obtained from previous objective using advanced oxidation processes. A pilot scale study has been conducted on orifice embedded hydrodynamic cavitation reactor for treating real-time TWE. Liquid chromatography-high resolution mass spectrometry (LC-HRMS) has been carried out to identify the specific compounds present in TWE before and after treatment. The common compounds in TWE were found to be phenolic compounds (p-nitrophenol), 4-chloro-m-cresol, polyethylene glycols (PEGs), dibutyl phthalate (DBP), nonyl phenol ethoxylated surfactants and alcohol ethoxylated surfactant. The mass spectra reflected that several intermediate peaks of aromatic compounds, i.e., p-nitrocatechol (m/z=155.1102) and hydroquinone (m/z=110.1123) and acids, i.e., malic acid (m/z=134.0874), the carboxylic acid (m/z=150.150), malonic acid (m/z=104.0615), oxaloacetic acid (m/z= 134.0874) were found in the treated effluent. The spectrum shows that the major peaks found for raw TWE were reduced to less than 30%-50% approximately using a combination of HC+PAA+FeSO47H2O. Moreover, the experimental analysis done using Response surface methodology (RSM) – Box-Behnken design (BBD) obtained maximum COD, TDS and turbidity removal efficiency of 72.36, 94.54 and 98.89% respectively at cavitation time-97.5 min, inlet pressure-7.5 bar, pH-2, orifice diameter-0.5 mm, PAA dosage-17.5 g/L, and PAA /FeSO4.7H2O-2:1 ratio using. Kinetic studies resulted in increase of the reaction rate constant from 2.455x10-3 to 12.75x10-3 min-1 when AOPs were combined with HC and concluded that the combined system shows a synergy. The study concluded that the degradation using HC+PAA+ FeSO4.7H2O successfully increased the biodegradability of the TWE without forming any toxic by-products. In addition, the energy and cost requirements for the treatment system have also been discussed in detail to see the significant perspective for scaling – up the system at an industrial scale. Objective 3 – The increase in the biodegradability of the tannery wastewater using pre treatment has enhanced the potential of contaminants to produce biogas. The evaluation of biogas production by conducting batch scale study on mono-digestion and co-digestion of tannery wastewater effluent was performed in this study. The effect of influential parameters i.e., C/N and pH on the biogas production and COD reduction was determined. The pre treatment has shown an increase in biogas yield by 3 times compared to raw TWE using mono vii digestion. However, the co-digestion using chicken manure as a co-substrate resulted in 40.59 ml/g.COD and 104.75 ml/g.COD of biogas production using raw and hydrodynamic cavitation treated wastewater respectively at C/N ratio and pH of 20 and 7.5. Maximum COD removal of 64% at C/N-20 and pH-7.5 was observed for hydrodynamic cavitation treated wastewater. The study proved that the hydrodynamic cavitation as a pre-treatment resulted in significant increase in biogas production for mono-digestion as well as co-digestion. The biogas yield is more in co-digestion with chicken manure when compared to mono digestion. Objective 4 – This study has been conducted to determine the potential of leather trimmings used as a co-substrate to enhance methane yield. The effect of C/N and pH on the anaerobic digestion of substrates were evaluated to obtain improved biogas yields. The results have clearly shown that the ternary combination of the substrates has increased the methane yield substantially. The maximum of 300.75 ml/g.COD has been produced using leather trimmings as a co-substrate when the HC+PAA+FeSO4.7H2O (AOPs) treated wastewater was subjected to anaerobic digestion. However, the ternary combination i.e., AOP treated TWE, CM and LT has resulted in maximum methane yield of 489.81 ml/g.COD at C/N – 20 and pH – 6.5 respectively. In addition, the maximum COD reduction of 72% was obtained for the reactor operating at C/N 20 and pH-6.5. The study proved that the solid waste generated at tanning industry can successfully increase methane production if used as a co-substrate. Summary –The hybrid treatment system was able to remove suspended solids and degrade complex compounds present in TWE into simpler compounds resulting in increase of biodegradability index. The increase in the biodegradability of the contaminants present in tannery wastewater effluent increased the methane production potential. Moreover, the solid waste generated from the tanning industry successfully increased the methane production potential when used as a co-substrate. The research study concluded that the series of combination of treatment systems was able to treat tannery wastewater up to an extent where it can be further treated aerobically.